2.1.

Materials

We synthesized the tin-oxo cage with two tosylates as the anions (TinS) according to a procedure described by Eychenne-Baron et al.¹⁸ Butylstannoic acid hydrate ($\mathrm{BuSnOOH}•{\mathrm{xH}}_2\mathrm{O}$) was mixed with an excess of p-toluene sulfonic acid monohydrate ($\mathrm{pTsOH}•{\mathrm{H}}_2\mathrm{O}$) in a round bottom flask with toluene as a solvent. The reaction mixture was refluxed overnight under removal of water using a Dean–Stark apparatus. The resulting product was filtered using Celite to remove insoluble material. The toluene was then removed under reduced pressure to yield a white crude product, which was further purified by recrystallization from 1,4-dioxane. Needle-shaped white crystals were isolated.

The dihydroxide form of the tin cage (TinOH) was obtained using a procedure described by Eychenne-Baron et al.¹⁸ A 20 wt. % solution of TinS in isopropanol (IPA) was mixed with a solution of aqueous tetramethylammonium hydroxide (TMAH) in IPA. The TMAH was used in excess ($2.3\times$). Precipitation occurred instantaneously. The resulting suspension was filtered to obtain a white powder.

The diacetate (TinA) and malonate (TinM) forms of the tin cluster were obtained using a procedure described by van Lokeren et al.¹⁹ The dihydroxide TinOH was dissolved in tetrahydrofuran (UvaSol, spectroscopic grade) after which two molar equivalents were added of glacial acetic acid (Aldrich), or one molar equivalent of malonic acid (purified by recrystallization), respectively. NMR analysis was carried out using a Bruker AV-400 NMR spectrometer. The ${}^1\mathrm{H}$ and ${}^{119}\mathrm{Sn}$ NMR spectra were found to be in agreement with the literature.^10,12,18 The thermogravimetric analysis (TGA) of TinOH was performed using a NETZSCH thermogravimetric analyzer, using 22.2 mg of sample in an ${\mathrm{Al}}_2{\mathrm{O}}_3$ crucible. Heating was performed from 35°C to 800°C at $10\mathrm{K}/\min$ in an $80\%/20\%{\mathrm{N}}_2/{\mathrm{O}}_2$ atmosphere.

2.2.

Preparation of Thin Films

For a typical thin film of $\sim 40\mathrm{nm}$ thickness, a solution is made of $15\mathrm{mg}/\mathrm{mL}$ of the tin-oxo cage (with any of the counterions) dissolved in toluene using sonication (30 s). The malonate form did not sufficiently dissolve in toluene and was therefore dissolved in methanol. Particulates were removed by filtering the solutions using a $0.2\text{-}\mu \mathrm{m}$ syringe filter before spin coating. The solutions were spun on Si wafers (for interference lithography) and $2\times 2{\mathrm{cm}}^2$ Si chips (for open-frame experiments) with a spin speed of 2500 rpm for 45 s, with a subsequent soft bake of 30 s at 90°C. Both wafers and chips were treated with hexamethyldisilazane (HMDS) before spin coating. After exposure, the samples were developed using a $2:1$ $\mathrm{IPA}/{\mathrm{H}}_2\mathrm{O}$ mixture for 30 s and rinsed for 10 s.

2.3.

EUV Exposure

EUV exposures were carried out at the XIL-II beamline of the Swiss Light Source (SLS) synchrotron at the Paul Scherrer Institute (PSI) with EUV light at $\lambda =13.5\mathrm{nm}$.²⁰ For the open-frame experiments, $0.5\times 0.5{\mathrm{mm}}^2$ areas were exposed to EUV light through a square aperture. For the patterning experiments, a transmission mask was used providing line/space patterns with pitches of 100, 80, 60, and 44 nm.

2.4.

Postexposure Analysis

Atomic force microscopy (AFM) images were made using a Bruker Dimension Icon, using the PeakForce tapping (ScanAsysAir) mode. Perpendicular to the resist edge, fields of $40\times 5{\mu \mathrm{m}}^2$ were scanned with $128\text{samples}/\text{line}$, using a scan speed of 0.5 Hz. The raw images were corrected for bowing using first- and second-order corrections, and the film thickness was measured by comparing the height of the film with the height of the substrate, as shown in Fig. 2. Scanning electron microscopy (SEM) imaging was performed using a Carl Zeiss SUPRA 55VP SEM with a voltage of 1 kV.

Fig. 2

AFM image of the material edge (bottom). Image height was corrected using a second-order flattening procedure. The height values of the image were then averaged along the $y$-axis to produce an edge cross section (top). The average substrate height (red) was compared with the average photoresist height far from the edge (blue).

3.1.

Open-Frame Experiments: Contrast Curves

Contrast curves (also known as H-D curves or characteristic curves) were obtained by measuring the remaining thickness after development as a function of EUV exposure dose using AFM imaging (see experimental section).

A typical AFM image of the material edge is shown in Fig. 2. Here, the left part of the image is blank Si substrate, because the unexposed material was removed by the developer. The right part of the image shows exposed and therefore insoluble material. It can be seen that the edge of the profile is partly covered by a wave pattern, caused by interference effects (knife-edge effect). To measure the remaining thickness correctly, the height of the resist was measured as far from the edge as possible and averaged over a certain distance window.

Contrast curves were first used to compare compounds with different counterions, TinOH and TinA. The anion of the tin cage can not only alter the reaction of the material to EUV excitation¹⁰ but also its physical properties such as solubility. For instance, doubly charged anions such as malonate combined with the cage compound render the material more polymeric in nature because the tin cage can form elongated chains with the doubly charged counterion.¹⁹

TinOH and TinA were spin coated with a subsequent postapplication bake (90°C) and exposure (see experimental). They did not differ significantly in their performance judging from the contrast curves (Fig. 3). This means that the anions are probably not directly involved in the mechanism that leads to the solubility switch. They could act as nonreactive spacers, as was proposed by Cardineau et al.¹⁰ We can observe the onset of gelation at $\sim 10\mathrm{mJ}/{\mathrm{cm}}^2$ and a maximum thickness at $\sim 50\mathrm{mJ}/{\mathrm{cm}}^2$. At higher doses, the thickness decreases again. This is likely due to loss of more of the butyl chains, in analogy to the results of deep UV exposures.¹⁶

Fig. 3

Remaining layer thickness as a function of EUV dose for TinOH and TinA. Initial thickness was $\sim 40\mathrm{nm}$. No PEB was applied.

In addition to the anion effect, improvements to the process can also alter the contrast curves. In fact, photoresist parameters such as sensitivity and resolution are only meaningful concepts if the process conditions are optimized. Parameters that were optimized were: developer, development time, PAB, and PEB. First, the use of a different developer may improve resist performance because of a larger difference in the solubility rates of the exposed and unexposed areas. A few other developers instead of the $2:1$ $\mathrm{IPA}/{\mathrm{H}}_2\mathrm{O}$ mixture¹⁰ were tried, such as 2-heptanone.²¹ None of them, however, seemed to significantly improve resist performance. We will therefore only show results obtained with the $2:1$ $\mathrm{IPA}/{\mathrm{H}}_2\mathrm{O}$ developer.

Another important processing step is postexposure baking (PEB). This is well known for CARs: heating the resist after exposure allows diffusion of the photoacids and induces the removal of the dissolution inhibiting protecting groups.²² For non-CARs, this connection is less clear. Although photoacid diffusion does not apply here, the heating could still induce additional chemical reactions of species that were formed during the exposure step. This is especially the case since EUV exposures take place in vacuum. The photoresist material is baked under ambient atmosphere, enabling reactions of reactive species with oxygen and moisture.

However, the conditions of the PEB should be carefully tuned, since the unexposed material may also change solubility. This can lead to loss of contrast. In the case of a negative tone resist, it is essential that the unexposed part can still be fully cleared by the developer.

The baking effect was studied for two different anions: the hydroxide and the malonate. The malonate form is more polymeric in nature and may, therefore, have a different response to heating. The effect of baking at 100°C for the hydroxide form is shown in Fig. 4.

Fig. 4

Contrast curves of TinOH, measured without PEB (black) and with PEB at 100°C (red). Initial thickness was $\sim 40\mathrm{nm}$.

PEB (100°C, 2 min) has a significant effect on the sensitivity of the tin-oxo cage material. A lower dose is required to reach the onset of gelation (7 mJ/cm² PEB, 8 mJ/cm² non-PEB) and to achieve a remaining thickness of 20 nm, the dose needed is $2\times$ smaller. Additionally, the contrast seems to increase as the initial increase of remaining thickness with dose is $\sim 4\times$ larger for the baked samples. However, we note that the contrast measured in this way can be different from the theoretical contrast, which is defined as the difference in solubility rate between the exposed and nonexposed parts of the resist.²³ Measuring this more directly, for instance using a quartz crystal microbalance (QCM) would be needed to get an accurate value for the contrast.

The effect of baking was also investigated for a layer with higher initial thickness (60 nm instead of 40 nm). The baking effect was consistent here (see Fig. 5). However, we also note a large increase in the sensitivity for a thicker layer of TinOH. Only $2\mathrm{mJ}/{\mathrm{cm}}^2$ was needed for a reasonable remaining thickness (26 nm). Calculations by others show that a reasonable dose at the bottom is required for this type of materials, in order to make the converted material attach to the substrate.²⁴ However, these results contradict this. Apparently, converted material attaches easily even if it is not in direct connection with the substrate. A thicker layer leads to a larger amount of converted material, which apparently leads to a higher remaining thickness as well. The effect of resist thickness was investigated earlier for CARs in previous work,²⁵ which also showed dependence of lithographic performance on layer thickness. For CARs, pattern collapse occurred more often upon increasing layer thickness (25 to 30 nm), though on the other hand, LER decreased. Pattern quality also worsened for tin cage material upon increasing layer thickness (results not shown).

Fig. 5

The PEB effect for TinOH with a higher initial layer thickness (60 nm).

The films were also postexposure baked at two higher temperatures: 120°C and 150°C. The unexposed parts of the film became partly insoluble. In the case of 120°C, the unexposed resist layer was still lower in height than the exposed material, except at very low doses ($<5\mathrm{mJ}/{\mathrm{cm}}^2$). We suspect that, at these low doses, the shrinkage of the material was a larger factor than the solubility change. In the case of a 150°C PEB, the unexposed layer was barely soluble, leading to a decrease in height of the exposed parts for all used doses (2 to $50\mathrm{mJ}/{\mathrm{cm}}^2$). PEB at these higher temperatures is probably not suitable for patterning.

The effect of temperature on the materials was investigated by means of TGA, with which mass loss as a function of temperature can be measured. This can give additional information about the temperature stability of the unexposed material. A TGA curve of TinOH is shown in Fig. 6.

Fig. 6

TGA analysis for TinOH powder, in an 80%/20% ${\mathrm{N}}_2/{\mathrm{O}}_2$ atmosphere.

It can be seen that the remaining mass of TinOH first has a slight drop at the onset of the curve ($<100°\mathrm{C}$). This can be attributed to the loss of some loosely attached IPA, the solvent that was used in the conversion step from TinS to TinOH (see experimental section). The remaining mass reaches a first plateau around 100°C, but drops suddenly above 150°C, at which temperature we observe a change in solubility. Banse et al.²⁶ reported that the crystal structure of TinOH contains four molecules of IPA, but they observed that powders are easily obtained that contain less IPA. The weight loss at $\sim 150°\mathrm{C}$ could be due to the loss of the most tightly bound solvent. It is also conceivable that water is eliminated, accompanied by deprotonation of the bridging OH-groups. Above $\sim 200°\mathrm{C}$, a gradual loss of $\sim 22\%$ of the original weight occurs that can be attributed to the loss of organic content (cleavage of the butyl groups). The final remaining mass is 72%, which is close to the theoretical remaining mass of 73% assuming complete conversion to ${\mathrm{SnO}}_2$. The slightly lower remaining mass can be attributed to the presence of IPA or ${\mathrm{H}}_2\mathrm{O}$ in the initial sample.

Temperature response can be different for different tin cage materials. The malonate form (TinM) was also baked at 120°C. Surprisingly, the nonexposed TinM was still completely soluble even at this temperature. In addition, the sensitivity of the material seems to be high (see Fig. 7), although the high initial layer thickness also plays a role. If we compare the results to TinOH (Fig. 5), we see that in both cases a higher initial thickness leads to a higher remaining thickness. However, the measured contrast of TinM under these conditions appears to be lower than that of TinOH (PEB at 100°C, Fig. 5), which could explain why the patterning results of TinM were not significantly better.

Fig. 7

Contrast curve of TinM, using a PEB of 120°C. Initial thickness was $\sim 50\mathrm{nm}$.

3.2.

Interference Lithography Experiments: Patterning

Line patterns were printed in $\sim 30\text{-}\mathrm{nm}\text{-}\mathrm{thick}$ films of the tin cage materials using EUV interference lithography. Transmission diffraction gratings on a ${\mathrm{Si}}_3{\mathrm{N}}_4$ membrane were used (mask) to create mutually coherent beams, which in turn interfere to form the desired interference pattern. In our case, by utilizing two gratings we obtained line/space patterns with half the periodicity of the gratings on the mask.²⁷ Patterning experiments are important to demonstrate the potential of materials for photoresist applications: a performance is sought with the resolution and sensitivity being as high as possible, and the line edge roughness being minimized.

In Fig. 8, the effect of PEB can be seen. The pattern is almost invisible for the $34\mathrm{mJ}/{\mathrm{cm}}^2$ pattern, but clearly present when a postexposure bake (100°C, 30 s) is applied before development. In the case of the $131\mathrm{mJ}/{\mathrm{cm}}^2$ patterns, the lines increase in width with respect to the spaces. The lines are around 50 nm for the non-PEB sample but increase in width to around 68 nm for the PEB sample. This must mean that kinetically stable reaction products are formed during the EUV exposure, which reacts further during the PEB step.

Fig. 8

The effect of PEB on TinOH measured at two different doses (34 and $131\mathrm{mJ}/{\mathrm{cm}}^2$) for 50-nm half-pitch lines.

Other parameters that were investigated were rinsing time and hard baking (postdevelopment baking). These parameters were investigated because they could have a favorable effect on microbridging, an undesirable phenomenon that was encountered during interference lithography experiments. The effect of rinsing and hard bake on this phenomenon is shown in Fig. 9.

Fig. 9

Minimization of bridging effect, for TinOH using a dose of $110\mathrm{mJ}/{\mathrm{cm}}^2$ on 50-nm HP lines. The regular rinsing time was 10 s. PEB was performed at 100°C for 2 min for all three patterns. Hardbaking (postdevelopment baking) was performed at 200°C for 1 min.

The images show that the bridging effect is reduced if a longer rinsing time is used. Longer rinsing time also leads to smaller lines relative to the spaces. Under standard conditions, the lines are around 60 nm, whereas longer rinsing decreases this to around 50 nm. This implies that the resist is still partially soluble in the rinser (${\mathrm{H}}_2\mathrm{O}$) and that the rinsing should not be performed for too long of a time. Hard baking (200°C for 1 min) does not significantly change the pattern, although it slightly reduces the line width to around 55 nm.

The materials were also tested at a higher resolution. Although 80-nm pitch lines could be printed, the 60- and 44-nm pitch lines were significantly more difficult to print. Significant bridging between lines was observed. Probably this has to do with a problem of the development or rinsing process. Pattern collapse does not seem very likely, however, since the aspect ratio of the pattern is close to $1:1$. Rather, the lines seem to detach from the substrate, signaling poor adhesion of the resist film or pattern collapse. For none of the presently studied materials, patterns at 22-nm HP could be resolved. Further investigations and improvements are needed to reach high resolution, such as reducing the aspect ratio, changing and optimization of the rinsing solvent, and improving adhesion of the resist to the substrate using surface treatments or underlayers.

Similarly to the open-frame experiments, we also patterned thicker layers (30 nm instead of 20 nm). However, a higher sensitivity was not found here. A reason could be that adhesion of the lines is very important for patterning experiments. A thicker layer receives less light at the bottom, making this adhesion more difficult.

The best patterns were created for the tin cage with acetate (${\mathrm{CH}}_3{\mathrm{COO}}^{-}$) anions. This material was significantly less prone to pattern collapse or bridging, perhaps owing to its better stability. Two patterns at half pitches of 40 and 30 nm for this anion can be seen in Fig. 10. As it is seen in the figure, patterning with high quality is obtained down to 30-nm HP and the resolution is limited by the pattern collapse due to the adhesion problems.

Fig. 10

Interference lithography on TinA. Processing conditions: PAB 90°C (1 min), PEB 100°C (1 min), and hard-bake 200°C (1 min).